A fluorocopolymer (hereinafter referred to simply as “fluororesin”) such as polytetrafluoroethylene, a tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer or an ethylene/tetrafluoroethylene copolymer, has characteristics excellent in e.g. heat resistance, chemical resistance, water resistance, oil resistance, weather resistance, aging resistance, gas barrier properties, fuel barrier properties, release properties, non-stickiness, antifouling properties, dye adhesion resistance and unelution properties, and such a fluororesin is employed in various fields such as semiconductor industries, aircraft or automobile industries, food manufacturing industries and medical industries. The fluororesin has the above characteristics, and thus is basically preferably used for transportation tubes for e.g. pure water, ultrapure water, drinking water, alcoholic beverages such as beers, nonalcoholic beverages such as juices, pharmaceuticals, blood, transfusion or chemicals.
Accordingly, in the case of employing such a fluororesin for the above application, it has been attempted to make it into a laminate with e.g. other general-purpose resin materials in order to compensate the high cost and lack in mechanical strength such as abrasion resistance, toughness or flexibility which is further required to be provided. However, a fluororesin is basically poor in adhesion to other materials, and therefore, it was difficult to make it into a strong laminate. For example, similarly to a case where the fluororesin is bonded to e.g. a metal, a metal oxide, glass or ceramics, also in a case where it is bonded to other resin materials, e.g. a method has been employed in which the surface of the fluororesin is subjected to corona discharge treatment, sodium etching treatment or the like, and then an adhesive is applied on the surface for bonding. Such a bonding method is cumbersome in process steps, low in productivity and high in cost of products. Further, in a method of etching a fluororesin tube by sodium to improve interlayer adhesion, the tube tends to be discolored, and therefore, there is such a problem that it is impossible to use the tube for the application where the transparency is required. Accordingly, it is desired to develop a fluororesin which can be bonded to other resin materials by a simpler method which replaces the above method.
Heretofore, from such a viewpoint, a method for producing a tube excellent in mechanical properties and fuel barrier properties at a low cost has been known, in which co-extrusion is carried out to laminate a polyamide and an ethylene/tetrafluoroethylene copolymer having an adhesive functional group such as an acid anhydride residue of itaconic acid or citraconic acid introduced. Such a tube has been utilized as a fuel tube for automobiles (see Patent Document 1).
It is disclosed in Patent Document 2 that, in order to improve the bonding strength of a tube obtained by laminating an outer layer of a polyurethane superior in flexibility to a polyamide and an inner layer made of a fluororesin such as an ethylene/tetrafluoroethylene copolymer, an inorganic filler is blended to the fluororesin and further a silane coupling agent is blended to the polyurethane so as to bond the filler to the silane coupling agent. However, by such a technique, the adhesion between the polyurethane and the fluororesin was still insufficient.
On the other hand, there has been a problem that the molding process temperature of a fluororesin is high at the time of laminating the fluororesin with other resin materials. Namely, the molding temperature of a usual fluororesin is usually from about 280 to 350° C., and most of thermoplastic resins cannot withstand such a high temperature molding condition. Further, as a fluororesin which is moldable even at a low temperature of at most 250° C., preferably less than 250° C., e.g. an ethylene/tetrafluoroethylene/hexafluoropropylene copolymer has been known (see Patent Document 3 and Patent Document 4).
Patent Document 5 discloses a method of bonding an ethylene/tetrafluoroethylene/hexafluoropropylene copolymer and a thermoplastic resin such as polyamide, polyurethane, an acid-modified ethylene/vinyl acetate copolymer or an ethylene/methyl acrylate copolymer by means of thermal lamination at a temperature of from 250 to 300° C. Further, Patent Document 6 discloses treatment of the surface of a fluororesin with an adhesive composition (primer) made of an amino-substituted organic silane which may contain a phase transfer catalyst, to improve the bonding properties with polyurethane or polyamide. It is disclosed that a fluororesin and a thermoplastic resin are bonded by such techniques, whereby it is possible to obtain e.g. a laminate tube having both the flexibility of the thermoplastic resin and the chemical resistance of the fluororesin. However, the above laminating methods take a considerable time for bonding, require process steps of applying and treating a primer made of an adhesive composition, have a problem that productivity is poor due to complex process steps, and are insufficient even in the bonding strength.
Further, Patent document 7 and Patent Document 8 disclose a method of laminating substrates of an ethylene/tetrafluoroethylene/hexafluoropropylene adhesive copolymer having a specific amount of a carbonate group and/or a carboxylic halide group introduced as an adhesive functional group, and e.g. a polyamide, an ethylene/vinyl alcohol copolymer or a modified-polyethylene. However, in such a method, it was difficult to increase the amount of the adhesive functional group to be introduced, and further, there was a problem that the adhesion durability of the laminate was not necessarily sufficient.
Patent Document 1: JP-A-2004-238405
Patent Document 2: JP-B-6-9917
Patent Document 3: Japanese Patent No. 3609866
Patent Document 4: Japanese Patent No. 3428026
Patent Document 5: JP-A-2000-516871
Patent Document 6: JP-A-2004-536722
Patent Document 7: WO 99/45044
Patent Document 8: WO 01/18142